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    nLF
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    nLF

    Normalised LF power (LF n.u.): relative share of low-frequency oscillations.

    nLF — normalised low-frequency power (LF n.u.) — expresses LF power as a percentage of the sum of LF and HF power. Normalisation removes the influence of total power, which differs widely between people, and shows the relative share of the slow (baroreflex-related) oscillations compared with the respiratory (vagal) oscillations in a given recording.

    Heart rate variabilityFrequency domainNormalised units
    FormulaLF / (LF + HF) × 100
    Unitn.u.
    Healthy adults, 5 min52 ± 10 n.u.
    Usual range≈ 30–65 n.u.

    How it is calculated

    LF n.u. = LF / (TP − VLF) × 100 = LF / (LF + HF) × 100

    Because VLF is excluded, LF n.u. and HF n.u. add up to 100 (LF n.u. + HF n.u. = 100). They therefore carry the same information as the LF/HF ratio, expressed on a percentage scale. Normalised units are widely used in sleep research and in studies of posture and mental load.

    Power spectrum of heart rate variability with very low frequency band up to 0.04 hertz, low frequency band 0.04 to 0.15 hertz and high frequency band 0.15 to 0.40 hertz; total power is the area under the whole curve VLFLFHF ≈ 0.1 Hz: baroreflex≈ 0.25 Hz: breathing (vagus) 00.040.150.40 Frequency, HzPower, ms²/Hz
    Figure 1. Heart-rate power spectrum. LF n.u. is the share of the LF band (orange) in the combined LF + HF power; the VLF band is not included.

    Interpretation

    LF n.u.Relative patternTypical situations
    < 30Low — respiratory (HF) oscillations dominateDeep sleep, rest with strong vagal tone, fast breathing that shifts power to HF
    30–65UsualHealthy adults at rest (Nunan 2010)
    > 65High — slow oscillations dominateStanding, mental stress, REM sleep, slow breathing near 6/min; weak vagal modulation
    Interpret with caution

    A high LF n.u. does not necessarily mean high sympathetic activity: it can result simply from a fall in HF power (less vagal modulation) or from slow breathing. Normalised units should always be read together with the absolute LF and HF powers, breathing rate and heart rate.

    When to see a doctor

    nLF alone is not a diagnostic test. Discuss persistent changes in spectral indices with a doctor if they accompany palpitations, dizziness, high blood pressure, sleep problems or chronic fatigue.

    References

    1. Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Heart rate variability: standards of measurement, physiological interpretation and clinical use. Circulation. 1996;93(5):1043–1065. doi:10.1161/01.CIR.93.5.1043
    2. Nunan D, Sandercock GRH, Brodie DA. A quantitative systematic review of normal values for short-term heart rate variability in healthy adults. Pacing Clin Electrophysiol. 2010;33(11):1407–1417. doi:10.1111/j.1540-8159.2010.02841.x
    3. Billman GE. The LF/HF ratio does not accurately measure cardiac sympatho-vagal balance. Front Physiol. 2013;4:26. doi:10.3389/fphys.2013.00026

    This article is for general information and does not replace a consultation with a doctor. HRV values depend on the device, recording length and conditions.

    Published on 24 June 2019